U.S. patent number 6,493,555 [Application Number 09/199,194] was granted by the patent office on 2002-12-10 for method of improving cooperation between entities during call handover.
This patent grant is currently assigned to Alcatel. Invention is credited to Michael Roberts, Stephane Saada.
United States Patent |
6,493,555 |
Saada , et al. |
December 10, 2002 |
Method of improving cooperation between entities during call
handover
Abstract
A method of improving co-operation between entities in a
cellular mobile radio communications network, wherein during
inter-cell call handover, a new server cell is chosen from a set of
candidate cells as being a candidate cell to which a requested
handover is possible, and candidate cells to which a requested
handover is not possible are referred to as rejected cells. When
executing an external call handover from a current server cell
controlled by a current server entity to a new server cell chosen
from the set of candidate cells and controlled by a new server
entity which is distinct from the current server entity, the new
server entity is informed of any previously rejected cells.
Inventors: |
Saada; Stephane
(Fontenay-Sous-Bois, FR), Roberts; Michael
(Neuilly-sur-Seine, FR) |
Assignee: |
Alcatel (Paris,
FR)
|
Family
ID: |
9513864 |
Appl.
No.: |
09/199,194 |
Filed: |
November 25, 1998 |
Foreign Application Priority Data
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Nov 27, 1997 [FR] |
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97 14912 |
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Current U.S.
Class: |
455/438; 455/436;
455/525 |
Current CPC
Class: |
H04W
36/0055 (20130101); H04W 36/00835 (20180801); H04W
36/10 (20130101) |
Current International
Class: |
H04Q
7/38 (20060101); H04Q 007/20 () |
Field of
Search: |
;455/436,438,439,525,421,62,63,560 ;370/321 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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196 15 955 |
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Dec 1996 |
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DE |
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2 720 887 |
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Dec 1995 |
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FR |
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WO 97/44984 |
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Nov 1997 |
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WO |
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Primary Examiner: Chin; Vivian
Assistant Examiner: Ramos-Feliciano; Eliseo
Attorney, Agent or Firm: Sughrue Mion, PLLC
Claims
What is claimed is:
1. A method of improving co-operation between entities in a
cellular mobile radio-communications network, during inter-cell
call handover, the method comprising when executing an external
call handover from a current server cell controlled by a current
server entity to a new server cell selected from a plurality of
candidate cells and controlled by a new server entity which is
distinct from said current server entity, providing to said new
server entity information regarding previously rejected cells for
which a requested handover was not possible, wherein the
information regarding the previous rejected cells includes
information identifying the previously rejected cells.
2. A method according to claim 1, wherein said information
regarding the previously rejected cells includes information
regarding a reason why the requested handover was not possible, or
information regarding a time period after which a handover to said
previously-rejected cells can be requested again.
3. A method according to claim 1, wherein said step of providing to
said new server entity information regarding previously rejected
cells comprises selecting one of said candidate cells as a target
cell and transmitting to a target entity which controls the target
cell a handover request and information regarding the
previously-rejected cells.
4. A method according to claim 3, wherein said information
regarding said previously-rejected cells is transmitted by the
current server entity to the target entity via a higher-level
entity which provides a link between the current server entity and
the target entity.
5. A cell control entity for controlling a server cell in a
cellular mobile radio-communications network, the cell control
entity comprising: means for receiving a handover request for
executing a call handover from a current server cell to the server
cell controlled by the cell control entity and information
regarding previously-rejected candidate cells for which a requested
handover was not possible; means for transmitting, along with a
handover request for executing a call handover from the server cell
controlled by the cell control entity to a target server cell,
information regarding previously-rejected cells for which a
requested handover from the server cell was not possible; and means
for preventing transmission of handover requests for executing a
handover from the server cell to previously-rejected cells for
which a requested handover was not possible, wherein the
information regarding- previously-rejected candidate cells includes
information identifying the previously-rejected candidate
cells.
6. A cell control entity according to claim 5, wherein said
information regarding the previously rejected cells includes
information regarding a reason why a requested handover to the
previously rejected cells was not possible, or information
regarding a time period after which a handover to said
previously-rejected cells can be requested again, and the cell
control entity further comprises means for authorizing transmission
of a request for handover to one of said previously-rejected cells
after said time period has lapsed, or after a lapse of time that is
a function of the reason why the requested handover to the one of
the previously rejected cells was not possible.
Description
The present invention relates generally to cellular mobile
radiocommunications systems, more particularly to managing radio
resources in such systems, and even more particularly to managing
radio resources in such systems while handing over a call between
cells (referred to as "inter-cell call handover").
BACKGROUND OF THE INVENTION
With reference to FIG. 1, the general architecture is recalled of a
cellular mobile radiocommunications system such as, in particular,
the Global System for Mobile communications (GSM). Such a system
essentially comprises: a plurality base stations or "Base
Transceiver Stations" ("BTSs"), such as those referenced BTS1,
BTS2, BTS3, and BTS4; in the example shown, each of the base
stations is assigned to a respective cell, the cells being
referenced C1, C2, C3, and C4 (other examples naturally being
possible in which the same BTS is assigned to a plurality of
cells), and the base stations co-operate with mobile stations, such
as MS1, MS2, MS3, MS4, situated in the cells; a plurality of "Base
Station Controllers" ("BSCs"), such as those referenced BSC1 and
BSC2, each of the BSCs controlling a respective subset of cells
from the point of view of managing the radio resources; in the
example shown, the Base Station Controller BSC1 controls the cells
Cl and C2, and the Base Station Controller BSC2 controls the cells
C3 and C4; and a plurality of "Mobile Switching Centers" ("MSCs"),
such as that referenced MSC1, each of the MSCs co-operating with
external networks, such as, in particular, Public Switched
Telephone Networks (PSTNs) or Integrated Services Digital Networks
(ISDNs), and controlling a subset of Base Station Controllers from
the point of view of managing calls; in the example shown, the
Mobile Switching Center MSC1 controls the Base Station Controllers
BSC1 and BSC2.
In such a system, a decision to hand over a call is made by the
network on the basis of a certain number of parameters, such as, in
particular, the results of measurements taken on radio signals for
the current server cell and for cells neighboring the current
server cell.
The cell to which the call is to be handed over may be referred to
as the "new server cell", and it is more precisely chosen from a
set of cells referred to as "candidate cells" (determined by the
network from said neighboring cells on the basis of said
measurement results) in the following manner.
Firstly a request is made to hand over the call to the best of the
candidate cells. When such handover is possible, the best candidate
cell constitutes the new server cell. When such handover is not
possible, a request is made to hand over the call to the following
candidate cell, in the order of preference of candidate cells, and
so on.
A candidate cell to which handover is requested may also be
referred to as the "target cell".
A target cell to which a requested handover is not possible is also
referred to as a "rejected cell". A target cell can be rejected in
particular because the mobile station does not succeed in setting
up a link with the network in the target cell, or else because no
radio resource is available in said target cell, because of traffic
overload, or because a testing or maintenance operation is in
progress in the target cell, this list naturally not being
exhaustive.
When the target cell is controlled by the same BSC as the current
server cell (in FIG. 1, this situation corresponds, for example, to
handover being requested from cell C1 to cell C2, or from cell C3
to cell C4, such handover also being referred to as "internal
handover" or "intra-BSC handover"), the BSC in question is capable,
by itself and internally, of determining whether or not handover to
the target cell is possible, i.e. whether or not the target cell is
a rejected cell, since said BSC manages the radio resources for
both of the cells.
When the target cell is controlled by a BSC (referred to as the
"target BSC") other than the BSC (referred to as the "current
server BSC" or more simply below as the "server BSC") that controls
the current server cell (also referred to more simply below as the
"server cell"), this situation corresponding for example, in FIG.
1, to a request to hand over a call from one or other of the cells
C1 and C2 to one or other of the cells C3 and C4 (such handover
also being referred to as "external handover", or "inter-BSC
handover"), the server BSC is not capable itself and internally of
determining whether or not handover to the target cell is possible,
i.e. whether or not the target cell is a rejected cell, and the
procedure is then as shown in one or other of FIGS. 2 and 3,
namely: the server BSC, referenced BSC.sub.s, firstly sends a
handover request message (referenced HO REQ in this example for
"handover required") to the MSC; the MSC in turn sends a message of
the same type (referenced HO REQ' in this example for "handover
request") to the target BSC, referenced BSC.sub.c ; after having
determined internally whether or not such a handover to the target
cell is possible, i.e. whether or not said target cell is a
rejected cell, the target BSC sends to the MSC either, when the
target cell is not a rejected cell (FIG. 2), a handover request
acknowledgment message (referenced HO REQ ACK in this example)
itself containing a handover command message, or else, when the
target cell is a rejected cell (FIG. 3), a message indicating that
it is impossible to perform handover to said target cell (this
message being referenced HO FAIL in this example for "handover
failure"); and the MSC in turn sends a message of the same type to
the server BSC, namely either a handover command message
(referenced HO COMM in this example), or else a message indicating
that it is impossible to perform handover to said target cell (this
message being referenced HO REJ for "handover request
rejected").
As shown in FIGS. 4 and 5, once a handover (or "transfer") T.sub.1
from one cell C.sub.i to another cell C.sub.j has actually been
performed, it is possible that a request might be made for a
handover T.sub.2 to a cell C.sub.k that was rejected previously
during the handover T.sub.1. If the cell C.sub.k was rejected
previously, then a handover to it was requested previously, i.e. it
was probably one of the best candidate cells, and it is thus
normal, in particular if the request for handover T.sub.2 is made
soon after handover T.sub.1, that it reappears as one of the best
candidate cells for handover T.sub.2, and thus that handover to it
is requested once again.
As shown in FIG. 4, when both of the cells C.sub.i and C.sub.j are
controlled by the same BSC, referenced BSC.sub.ij, the BSC is aware
of that fact that the cell C.sub.k was rejected during handover
T.sub.1, and is thus able to decide that, temporarily, a request
for such a handover T.sub.2 to cell C.sub.k must not be made.
As shown in FIG. 5, when the two cells C.sub.i and C.sub.j are not
controlled by the same BSC, the current server BSC, referenced
BSC.sub.j, is not aware of the fact that cell C.sub.k was rejected
during handover T.sub.1 (because only the initial server BSC,
referenced BSC.sub.i, is aware of that fact), and it is thus not
able to decide that, temporarily, such a request for handover
T.sub.2 to cell C.sub.k must not be made.
Thus: if the cells C.sub.j and C.sub.k are not controlled by the
same BSC, then the current server BSC (BSC.sub.j) uses the
procedure recalled above with reference to FIGS. 2 and 3 to send a
handover request to the target BSC (not shown) controlling the cell
C.sub.k, and it then might receive from the target BSC a message
indicating that it is impossible to perform handover to the cell
C.sub.k. An unnecessary handover request will then have been made,
thereby lengthening handover time unnecessarily, and giving rise to
unnecessary consumption of transmission resources for the purpose
of interchanging the corresponding signalling messages, and thus
degrading the quality of service; and if the cells C.sub.j and
C.sub.k are controlled by the same BSC, the current server BSC
(BSC.sub.j) might unnecessarily and internally have to determine
whether a handover to the cell C.sub.k is possible, thereby
lengthening call handover time unnecessarily, and thus also
degrading the quality of service.
OBJECTS AND SUMMARY OF THE INVENTION
A particular object of the present invention is to avoid those
drawbacks, i.e. to optimize operation of the network in such cases,
and more generally to improve co-operation between entities of the
network during inter-cell call handover.
The present invention provides a method of improving co-operation
between entities in a cellular mobile radio-communications network,
during inter-cell call handover, during which a "new server cell"
is chosen from a set of "candidate cells" as being a candidate cell
to which a requested handover is possible, candidate cells to which
a requested handover is not possible being referred to as "rejected
cells", wherein, during "external" call handover, namely when
handing over a call from a "current server cell" controlled by a
"current server entity" to a "new server cell" controlled by a "new
server entity" which is distinct from said current server entity,
said new server entity is informed of any previously rejected
cells.
Thus, since the new server entity is informed of the
previously-rejected cells, it does not attempt unnecessarily to
make requests for handover to previously-rejected cells, thereby
avoiding the above-mentioned drawbacks.
According to another characteristic, said information as to any
previously-rejected cells further includes an indication as to the
reason for which the cells were rejected, or as to a lapse of time
after which a handover to said previously-rejected cells can, if
necessary, be requested again.
Thus, the new server cell is then able to determine the time at
which a request can, if necessary, be made again for handover to
the previously-rejected cells, thereby making it possible to
optimize network operation further.
The present invention also provides a cell control entity for a
cellular mobile radiocommunications network, in particular a BSC
for a network such as a network of the GSM type, for implementing
such a method.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects and characteristics of the present invention will
appear on reading the following description of an embodiment given
with reference to the accompanying drawings, in which:
FIG. 1 is a diagram showing the organization of a mobile
radiocommunications network such as, in particular, the GSM
network;
FIG. 2 is a diagram showing the various messages interchanged
between various entities of such a network during an external
handover request, and when such handover is possible;
FIG. 3 is a diagram showing the various messages interchanged
between various entities of such a network during an external
handover request, and when such handover is not possible;
FIGS. 4 and 5 are diagrams showing how the problem is solved by the
present invention;
FIG. 6 is a diagram showing an example of a method of the invention
for improving co-operation; and
FIG. 7 diagrammatically shows an example of means to be implemented
in accordance with the invention in a cell control entity for a
cellular mobile radio-communications network (in particular a BSC
for a GSM-type network).
MORE DETAILED DESCRIPTION
The present invention thus relates to a method of improving
co-operation between entities in a cellular mobile
radiocommunications network, during inter-cell call handover,
during which a "new server cell" is chosen from a set of "candidate
cells" as being a candidate cell towards which a requested handover
is possible, candidate cells to which a requested handover is not
possible being referred to as "rejected cells".
In accordance with the invention, to improve co-operation between
entities of the network during "external" call handover, namely
handover from a "current server cell" (such as cell C.sub.i shown
in FIG. 5) controlled by a "current server entity" of the system
(such as entity BSC.sub.i shown in FIG. 5) to a "new server cell"
(such as cell C.sub.j shown in FIG. 5) controlled by a control
entity that is distinct from said current server entity and that is
referred to as the "new server entity" (such as entity BSC.sub.j
shown in FIG. 5), said new server entity is informed of any
previously rejected cells (such as cell C.sub.k shown in FIG.
5).
For this purpose, in the implementation described with reference to
FIGS. 6 and 7, with a candidate cell to which handover is requested
being referred to as the "target cell", with the control entity
BSC.sub.c for controlling a target cell being referred to as the
"target entity", and with each target entity potentially becoming
the new server entity if the requested handover to the target cell
is possible, each target entity is informed, during a request for
handover to the target cell, of any previously-rejected cells.
In addition, in the implementation described with reference to
FIGS. 6 and 7, the target entity BSC.sub.c is informed of any
previously-rejected cells by the server entity BSC.sub.s.
In addition, in the implementation described with reference to
FIGS. 6 and 7,the information as to any previously-rejected cells
is transmitted from the server entity BSC.sub.s to the target
entity BSC.sub.c via the entity MSC (or "higher-level entity")
providing the link between the server entity and said target
entity.
Thus, in the diagram shown in FIG. 6, such information as to any
previously-rejected cells (referenced INF) is associated with the
message (HO REQ) transmitted to the MSC by the server BSC
(BSC.sub.s) using the procedure recalled above with reference to
FIGS. 2 and 3.
Similarly, the information is re-transmitted by the MSC to the
target BSC (BSC.sub.c), the information then being referenced INF'
and also being associated, in the example shown, with the
corresponding message (HO REQ') re-transmitted by the MSC to the
target BSC (BSC.sub.c).
In another implementation (not shown specifically), it is also
possible for each target entity to be informed of any
previously-rejected cells by the entity MSC rather than by the
current server entity. Such an implementation corresponds, in
particular, to the case when the current server entity transmits
the list of candidate cells to the MSC, and when the MSC itself
successively makes the corresponding handover requests.
Advantageously, the information itself then contains an indication
as to the reason for rejecting said cells, or as to a lapse of time
after which a handover to said cells can, if necessary, be
requested again.
It should also be noted that the previously-rejected cells can be
not only cells rejected during the handover in progress, but also
cells rejected during preceding handovers, regardless of whether
the handovers are internal handovers or external handovers.
FIG. 7 diagrammatically shows the type of means to be used, in an
implementation of the invention, in a cell control entity for a
cellular mobile radiocommunications network, in particular a BSC
for a GSM-type network.
In a manner that is known and therefore not described again in
detail herein, for the case when the controlled cell is constituted
by a server cell, such an entity BSC includes means 1 for
transmitting a handover request message (HO REQ message) requesting
handover to a target cell, and means 2 for receiving messages
obtained in reply to such a request (HO COMM and HO REJ messages),
these means 1 and 2 operating in known manner in association with
means 3 for managing radio resources.
In the example shown, for the case when the controlled cell is
constituted by a server cell, the entity BSC further includes means
4 for transmitting information INF as to any previously-rejected
cells. In the example shown, this information INF is transmitted in
association with the handover request message HO REQ.
For this purpose, the means 4 receive information issued by the
means 3 for managing radio resources and as to the list of cells
rejected during a handover in progress, and they put the
information they receive in a form suitable for it to be
transmitted to the entity MSC, so as to constitute the information
INF.
Optionally, cells rejected during preceding handovers, regardless
of whether they were external handovers or internal handovers, may
be added to the list in a way which is described below.
Furthermore, the information INF may itself include an indication
as to the reason for rejecting the cells in question, or as to a
lapse of time after which a handover to said cells can, if
necessary, be requested again. As discussed above, a cell can be
rejected for example because it is impossible for the mobile
station to set up a link with the network in the cell, or because
of traffic overload in said cell, or else because a testing or
maintenance operation is in progress in the cell, these examples
naturally not being limiting.
Also in a manner chat is known and therefore not described again in
detail herein, and for the case when the controlled cell is
constituted by a target cell, such an entity BSC includes means 5
for receiving handover request messages (HO REQ' messages) and
means 6 for transmitting either a handover request acknowledgment
message (HO REQ ACK message), or a message indicating that it is
impossible to perform handover to said target cell (HO FAIL
message), these means 5 and 6 operating, also in known manner, in
association with the means 3 for managing radio resources.
In the example shown, and for the case when the controlled cell is
constituted by a target cell, the entity BSC further includes means
7 for receiving information such as INF' as to any
previously-rejected cells. In the example shown, this information
INF' is received in association with the handover request message
HO REQ'.
The means 3 for managing radio resources are controlled by the
means 7 in a manner such that, when the controlled cell is
constituted by a cell that is successively a target cell and then a
server cell, the means 1 are prevented from transmitting requests
for handover to such previously-rejected cells.
Furthermore, when said information INF' includes an indication
relating to the reason for rejecting the cells in question, or to a
lapse of time after which handover to said cells can, if necessary,
be requested again, the means 3 for managing radio resources are
further controlled by the means 7 so as to authorize transmission
of such requests for handover to said cells after such a lapse of
time or after a lapse of time (e.g. a predetermined lapse of time)
which is a function of said reason.
When the rejected cells can be not only cells rejected during the
handover in progress, but also cells rejected during preceding
handovers, the list of such cells rejected during preceding
handovers, as obtained by the means 7 on the basis of the
corresponding information INF', is added by the means 3 for
managing radio resources to the list of cells rejected during the
handover in progress.
The means 1 for transmitting handover request messages (REQ
messages), the means 2 for receiving is messages obtained in reply
to such a request (HO REJ and HO COMM messages), the means 5 for
receiving handover request messages (HO REQ' messages), the means 6
for transmitting either a handover request acknowledgment message
(HO REQ ACK message) or a message indicating that it is impossible
to perform handover to a target cell (HO FAIL message), and, as
regards their functions other than those relating to the present
invention, the means 3 for managing radio resources may be means
that are conventional and that therefore do not need to be
described again herein.
The particular manner in which the means 4 for transmitting
information INF as to cells previously rejected, in which the means
7 for receiving corresponding information INF', and in which those
particular functions of the means 3 for managing radio resources
which relate to the present invention are implemented for the
purpose of enabling them to operate using the above-described
method poses no particular difficulty for the person skilled in the
art, and does not therefore need to be specially described
either.
It can be understood that the invention is not limited to the
above-described examples, and that, more generally, it makes it
possible to improve co-operation between entities in a cellular
mobile radiocommunications network, during "external" inter-cell
call handover.
* * * * *